Dynamic Characteristics Analysis Of The Electro-hydraulic System And Study On Speed Ratio Control For Dual State CVT

As the second generation Continuously Variable Transmission (CVT) developed since 1990s, dual state CVT makes full use of self-adaptive performance to load and over-load protection capacity of hydraulic torque converter, which improves starting performance, acceleration performance and low speed traveling performance apparently. Moreover, due to good properties of hydraulic torque converter such as automatic torque increase, speed variation, the continuously variable speed ratio variation range is extended, which ensures that the engine work on the more optimal fuel economy operation line so it can improve the vehicle fuel economy. Therefore, dual state CVT has been described as the main developmental direction of CVT in the future.This paper selects a new dual state CVT as the research object, which is developed by our group. Based on the previous efforts, the basic structure, working principle and transmission characteristic of the system are further confirmed. What's more, the electro-hydraulic control system is divided into six parts, the function of each subsystem is discussed, and the working principle and design idea of hydraulic control system are also analyzed. Thereinto, the dynamic characteristics of speed ratio and clamping force control subsystems are investigated mainly. First, the bondgraph model of clamping force control subsystem is established through bond graph modelling technique. The nonlinear state equations of the system are derived from the model and its dynamics are investigated. The simulation study identifies that some critical structure parameters have significant impacting on the transient respond of the system. Then, bondgraph modelling technique is used to investigate the dynamics of speed ratio control valve. Using valve nonlinear state equations derived from bondgraph model, the effect of some key structure and operating parameters on the dynamic characteristic of the valve is discussed. Finally, the dynamic performance of speed ratio control system is investigated according to the nonlinear mathematic model. Therefore, the theoretical foundation for design of the control valve in dual state CVT and analysis of CVT hydraulic systems is built by the research work above.On the basis of the nonlinear state equations of speed ratio control system, the transfer function for the system facing with practical controller design is gained. A DMC-PID cascade prediction control algorithm is designed for CVT speed ratio control. It is shown from simulation that the control algorithm accomplishes the better robustness and tracking performance than general PID. Besides, based on the speed ratio control system, the selection of control parameters in DMC is summarized through simulation study. The experiments of pressure response for proportional relief valve and proportional pressure reducing valve are completed, furthermore, the test of speed ratio step response with PID control when speed ratio occurs step changing is also accomplished. The results prove that the good correctness and precision of mathematic model, which can be used to analyze and predict the dynamic characteristics of valve for dual state CVT, and is feasible to verify the effectiveness of speed ratio control algorithm through simulation study.